Water-proof air-permeable filter and use of the same

ABSTRACT

A water-proof air-permeable filter ( 1 ) includes: a resin film ( 2 ) having formed therein a plurality of through pores ( 21 ); a treated layer ( 3 ) having hydrophobicity and oil repellency, and formed on at least one of both surfaces in the thickness direction of the resin film ( 2 ) such that the treated layer ( 3 ) has openings ( 31 ) at positions corresponding to the through pores ( 21 ); and a loop-shaped double-sided tape ( 4 ) stuck to an edge region of one of both surfaces in the thickness direction of the resin film ( 2 ), with the treated layer ( 3 ) interposed therebetween.

TECHNICAL FIELD

The present invention relates to a water-proof air-permeable filter tobe attached to, for example, a housing, and relates to use of thewater-proof air-permeable filter.

BACKGROUND ART

Conventionally, in, for example, automobile electric components such asautomobile ECUs (Electrical Control Units), motors, lamps, and sensors,household electric appliances such as electric toothbrushes, shavers,and cellular phones, and solar cells, an opening is provided in ahousing that accommodates an electronic component, a control board,etc., in order to eliminate a pressure difference between the inside andoutside of the housing, and the opening is covered with a water-proofair-permeable filter. This water-proof air-permeable filter ensuresventilation between the inside and outside of the housing and alsoprevents foreign matters such as water and dust from entering thehousing.

For such a water-proof air-permeable filter, a polytetrafluoroethylene(PTFE) porous membrane having favorable air permeability and high waterpressure resistance is commonly used (for example, see Patent Literature1). Generally, a PTFE porous membrane can be obtained by molding PTFEfine powder into a sheet shape and then stretching the sheet-shapedmolded body in two directions orthogonal to each other.

CITATION LIST Patent Literature

PTL 1: JP 2008-237949 A

SUMMARY OF INVENTION Technical Problem

In some cases, a water-proof air-permeable filter is stuck to a housingby means of, for example, heat welding, ultrasonic welding, or anadhesive agent. In view of improving work efficiency of the sticking tothe housing, a loop-shaped double-sided tape is preferably providedalong the edge region of the water-proof air-permeable filter.

However, in the case of a water-proof air-permeable filter using a PTFEporous membrane, if the stretch ratio at the time of producing the PTFEporous membrane is increased, the fiber diameter of the PTFE porousmembrane becomes small, which may lead to reduction in the area ofadhesion between the PTFE porous membrane and a double-sided tape andthereby lower the strength of adhesion therebetween. In particular, whensuch a water-proof air-permeable filter is placed in a high-temperatureenvironment, the strength of adhesion between the PTFE porous membraneand the double-sided tape greatly reduces, which may cause entry ofwater into a housing from the interface between the PTFE porous membraneand the double-sided tape.

In view of the above circumstances, an object of the present inventionis to provide a water-proof air-permeable filter that securely holds adouble-sided tape, and use of the water-proof air-permeable filter.

Solution to Problem

In order to solve the above problem, the present invention provides awater-proof air-permeable filter for ensuring ventilation and alsopreventing entry of water, the water-proof air-permeable filtercomprising: a non-porous resin film having formed therein a plurality ofthrough pores extending through the resin film in a thickness directionand each having a predetermined size larger than or equal to 0.01 μm andsmaller than or equal to 10 μm; a treated layer having hydrophobicityand oil repellency, and formed on at least one of both surfaces in thethickness direction of the resin film such that the treated layer hasopenings at positions corresponding to the plurality of through pores;and a loop-shaped double-sided tape stuck to an edge region of one ofboth surfaces in the thickness direction of the resin film, with thetreated layer interposed therebetween.

Here, the “size” of the through pore means the diameter of a circlehaving an area equal to the cross-sectional area of the through pore.

In addition, the present invention provides use of a water-proofair-permeable filter, the use comprising using the above-describedwater-proof air-permeable filter to cover an opening provided in ahousing, and eliminate a pressure difference between the inside andoutside of the housing via the water-proof air-permeable filter.

Advantageous Effects of Invention

With the above features, ventilation is allowed by the through poresformed in the resin film, and waterproofness can also be ensured by thetreated layer on the resin film. Furthermore, since the non-porous resinfilm is used, a large area of adhesion between the treated layer formedon the resin film and the double-sided tape can be ensured. Therefore,according to the present invention, the double-sided tape and thetreated layer can be adhered to each other with a sufficient adhesionstrength, and thus the double-sided tape can be securely held.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view of a waterproof air-permeablefilter according to one embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view of a water-proofair-permeable filter according to another embodiment of the presentinvention.

FIG. 3 is a diagram illustrating a method for measuring an adhesiveforce.

DESCRIPTION OF EMBODIMENT

Hereinafter, an embodiment of the present invention will be describedwith reference to the accompanying drawings.

FIG. 1 shows a water-proof air-permeable filter 1 according to anembodiment of the present invention. The water-proof air-permeablefilter 1 is for ensuring ventilation and also preventing entry of water.For example, the water-proof air-permeable filter 1 is attached to ahousing (not shown) so as to cover an opening provided in the housing,and is used for eliminating a pressure difference between the inside andoutside of the housing via the water-proof air-permeable filter 1.

Specifically, the water-proof air-permeable filter 1 includes anon-porous resin film 2, a treated layer 3 formed on the resin film 2,and a loop-shaped double-sided tape 4 provided along the edge region ofthe resin film 2. As used herein, the term “non-porous” means having asolid inside that is filled with a resin.

The shape of the resin film 2 is not particularly limited. For example,the resin film 2 may be circular or polygonal in a plan view (whenviewed in the thickness direction).

In the resin film 2, a plurality of through pores 21 extending throughthe resin film 2 in the thickness direction are formed. In other words,the through pores 21 are open at both surfaces in the thicknessdirection of the resin film 2. Typically, the through pores 21 arestraight pores having a given cross-sectional shape and extendinglinearly through the resin film 2. Such through pores can be formed by,for example, ion beam irradiation and etching. By use of ion beamirradiation and etching, through pores that have uniform sizes and axialdirections can be formed in the resin film 2.

The cross-sectional shape of the through pores 21 is not particularlylimited, and may be circular, or may be an undefined shape. In addition,the axial directions of the through pores 21 need not be a directionperpendicular to both surfaces in the thickness direction of the resinfilm 2, and may tilt relative to the direction.

The through pores 21 each have a predetermined size greater than orequal to 0.01 μm and smaller than or equal to 10 μm The sizes of thethrough pores 21 need not be exactly the same among all the throughpores 21, and it is sufficient that the through pores 21 have such sizesas can be considered to be substantially the same among all the throughpores 21 (for example, the standard deviation is 10% or less of theaverage value). The sizes of the through pores 21 can be adjusted byetching time or the concentration of an etching treatment liquid.Preferably, the size of each through pore 21 is 0.5 μm or more and 5 μmor less.

Preferably, the through pores 21 are uniformly distributed over theentire resin film 2 such that the density of the through pores 21 fallswithin specific limits included in a range from 10 to 1×10⁸ pores/mm²(for example, such that the maximum density is smaller than or equal to1.5 times of the minimum density). The density of the through pores 21can be adjusted by the number of ions applied at the time of ion beamirradiation. More preferably, the density of the through pores 21 is1×10³ to 1×10⁷ pores/mm².

The porosity of the resin film 2 (the proportion of the sum of thecross-sectional areas of all the through pores 21 to the area defined bythe outline of the resin film 2) is not particularly limited. In view ofensuring a film strength sufficient for the intended use, the porosityis preferably 50% or less, and more preferably 35% or less.

In addition, the thickness of the resin film 2 is not particularlylimited. In order to realize a structure that has small pore diameters(high water pressure resistance) and a high porosity (high airpermeability) (in order to form pores of small diameters even when athick base material is used), the ratio of the thickness of the resinfilm 2 to the predetermined size (T/D when the predetermined size isdefined as D and the thickness of the resin film is defined as T) ispreferably 1 or more and 10000 or less, and more preferably 5 or moreand 1000 or less.

The material of the resin film 2 is not particularly limited. Resinsthat can be decomposed by an alkali solution, an oxidant solution, or analkali solution containing an oxidant are preferred. For example, theresin film 2 is made of at least one resin selected from polyethyleneterephthalate (PET), polycarbonate (PC), polyimide (PI), polyethylenenaphthalate (PEN), and polyvinylidene fluoride (PVdF).

An etching treatment liquid such as an alkali solution and an oxidantsolution, which is appropriate for the material of the resin film 2, isused for the aforementioned etching for forming the through pores 21.For example, alkali solutions, such as potassium hydroxide and sodiumhydroxide, can be used as a solution for hydrolyzing the resin. Inaddition, for example, oxidant solutions, such as a chlorous acidaqueous solution, a hypochlorous acid aqueous solution, a hydrogenperoxide solution, and a potassium permanganate solution, can be used asa solution for oxidatively decomposing the resin. For example, when theresin film 2 is formed of any of PET, PEN, and PC, a solution containingsodium hydroxide as the main component is used as the etching treatmentliquid, and when the resin film 2 is formed of PI, a solution containingsodium hypochlorite as the main component is used as the etchingtreatment liquid. Furthermore, when the resin film 2 is formed of PVdF,a solution obtained by adding potassium permanganate to a solutioncontaining sodium hydroxide as the main component is used as the etchingtreatment liquid to decompose PVdF.

Alternatively, a membrane filter sold by Oxyphen AG or MilliporeCorporation can be used as the resin film 2 in which the through pores21 are formed.

The resin film 2 may not necessarily consist of a single layer, and mayconsist of a plurality of separate layers.

In FIG. 1, the treated layer 3 is formed on one of both surfaces in thethickness direction of the resin film 2. However, the treated layer 3may be formed on both surfaces of the resin film 2. That is, it issufficient that the treated layer 3 is formed on at least one of bothsurfaces in the thickness direction of the resin film 2.

Specifically, the treated layer 3 is formed in such a manner as to haveopenings 31 at positions corresponding to the through pores 21, and hashydrophobicity and oil repellency. Such a treated layer 3 can be formedby applying a hydrophobic oil repellent agent thinly onto the resin film2 and drying the agent. Examples of such an oil repellent agent includefluorine-based coating agents having a perfluoroalkyl group. Thethickness of the treated layer 3 is preferably smaller than half of theabove-described predetermined size of the through pores 21.

If an oil repellent agent is applied and dried on the resin film 2 inwhich the through pores 21 are formed as described above, the innercircumferential surfaces of the through pores 21 can also be coated witha second treated layer continuous with the treated layer 3. In thiscase, the sizes of the openings 31 of the treated layer 3 are smallerthan the sizes of the through pores 21 by the thickness of the secondtreated layer.

The double-sided tape 4 is stuck to the edge region of the surface ofthe resin film 2 that is covered with the treated layer 3, with thetreated layer 3 interposed therebetween. When the treated layer 3 isformed on both surfaces of the resin film 2, the double-sided tape 4 maybe stuck to either surface of the resin film 2. That is, thedouble-sided tape 4 is stuck to the edge region of one of both surfacesin the thickness direction of the resin film 2, with the treated layer 3interposed therebetween.

The double-sided tape 4 is produced by applying an adhesive to bothsurfaces of a base material. The base material and the adhesive are notparticularly limited. For example, a nonwoven fabric made of PET can beused as the base material, and an acrylic adhesive or a silicon-basedadhesive can be used as the adhesive.

For example, as shown in FIG. 2, an air-permeable supporting member 5may be laminated to the surface of the resin film 2 opposite to thesurface to which the double-sided tape 3 is stuck. As shown in FIG. 2,the air-permeable supporting member 5 may be laminated directly on theresin film 2. Alternatively, when the treated layer 3 is formed on bothsurfaces in the thickness direction of the resin film 2, theair-permeable supporting member 5 may be laminated to the resin film 2with the treated layer 3 interposed therebetween. The air-permeablesupporting member 5 preferably has better air permeability than theresin film 2. For example, a woven fabric, a nonwoven fabric, a net, amesh, or the like, can be used as the air-permeable supporting member 5.In addition, examples of the material of the air-permeable supportingmember 5 include polyester, polyethylene, and aramid resins.

The resin film 2 and the air-permeable supporting member 5 are joined bya common method such as heat welding and bonding using an adhesiveagent. The resin film 2 and the air-permeable supporting member 5 arepartially joined, and the area of the joined parts is preferably 5 to20% of the entire area. This is because when the area of the joinedparts is less than 5% of the entire area, the resin film 2 and theair-permeable supporting member 5 are likely to separate from eachother, whereas when the area of the joined parts is more than 20%, thewater pressure resistance at the joined parts is reduced. Preferably,the joined parts are distributed evenly over the entire area.

When a water pressure resistance is measured in the same manner as inJIS L1072-A (low water pressure method) or JIS L1072-B (high waterpressure method) after the water-proof air-permeable filter 1 having theabove features is stuck to a metal plate (e.g., stainless steel plate)having an opening in such a manner as to cover the opening, the waterpressure resistance is preferably 1 kPa or more and 1000 kPa or less.

In addition, when a measurement value is obtained by conductingmeasurement in the same manner as in JIS P8117 after the water-proofair-permeable filter 1 is stuck to a metal plate having an opening insuch a manner as to cover the opening, and a Gurley number is obtainedby converting the measurement value into a value per 642 mm², the Gurleynumber is preferably 0.5 seconds/100 mL or more and 500 seconds/100 mLor less.

The water-proof air-permeable filter 1 of the present embodiment allowsventilation by the through pores 21 formed in the resin film 2, and canalso ensure waterproofness by the treated layer 3 on the resin film 2.Furthermore, since the non-porous resin film 2 is used, a large area ofadhesion between the treated layer 3 formed on the resin film 2 and thedouble-sided tape 4 can be ensured. Therefore, in the water-proofair-permeable filter 1 of the present embodiment, the double-sided tape4 and the treated layer 3 can be adhered to each other with a sufficientadhesion strength, and thus the double-sided tape 4 can be securelyheld.

In conventional water-proof air-permeable filters using PTFE porousmembranes, a plurality of PTFE porous membranes are laminated in orderto increase the thickness in some cases. In this case, water may leakfrom an interface between the PTFE porous membranes during pressureresistance test due to a pressure lower than the water pressure thateach single PTFE porous membrane can withstand. By contrast, in the caseof the water-proof air-permeable film 1 of the present embodiment, it issufficient to use a thick resin film 2 in order to increase thethickness. Accordingly, no problem occurs during water pressureresistance test. In order to form the through pores 21 in the thickresin film 2, it is sufficient to irradiate the resin film 2 with heavyions at a high acceleration and a high density at the time of ion beamirradiation.

Furthermore, since PTFE porous membranes are made porous by stretching,a water-proof air-permeable filter using a PTFE porous membrane has alow tensile strength. By contrast, the water-proof air-permeable filter1 of the present embodiment is not stretched, and thus has a hightensile strength. That is, according to the water-proof air-permeablefilter 1 of the present embodiment, improvement in processability andresistance against external force can be expected.

In addition, with the features of the present embodiment, when theair-permeable supporting member 5 is used, the resin film 2 that is madeof the same material as the air-permeable supporting member 5 can beused. Therefore, for example, the adhesion between the air-permeablesupporting member 5 and the resin film 2 can be enhanced by improvingcompatibility at the time of thermal lamination.

EXAMPLES

Hereinafter, the present invention will be described in detail usingexamples. However, the present invention is not restricted by theexamples to any degree.

Example 1

A sample A of 23 μm thickness (Oxydisk manufactured by Oxyphen AG),which is produced by forming through pores with a diameter of 0.4 μm ina non-porous base sheet of PET by means of ion beam irradiation andetching, was used as a resin film.

A nonwoven fabric made of PET was laminated as an air-permeablesupporting member to one surface of the sample A which is a resin filmby heat welding, and then a fluorine-based treatment agent (X-70-029Cmanufactured by Shin-Etsu Chemical Co., Ltd.) was applied to the othersurface of the resin film and was dried, to form a treated layer havinghydrophobicity and oil repellency. From the laminated body thusproduced, a circular laminated body with a diameter of 10 mm was stampedout. Then, a ring-shaped double-sided tape which was produced byapplying an acrylic adhesive to both surfaces of a base material of PETand which had an inner diameter of 5.5 mm and an outer diameter of 10mm, was stuck to the edge region of the surface on the treated layerside of the stamped-out laminated body. In this manner, a water-proofair-permeable filter was obtained.

Example 2

A water-proof air-permeable filter was obtained in the same manner as inExample 1 except that a sample B of 22 μm thickness (Oxydiskmanufactured by Oxyphen AG), which is produced by forming through poreswith a diameter of 0.8 μm in a non-porous base sheet of PET by means ofion beam irradiation and etching, was used as a resin film.

Example 3

A water-proof air-permeable filter was obtained in the same manner as inExample 1 except that a sample C of 22 μm thickness (Oxydiskmanufactured by Oxyphen AG), which is produced by forming through poreswith a diameter of 1.0 μpm in a non-porous base sheet of PET by means ofion beam irradiation and etching, was used as a resin film.

Comparative Example 1

An adhesive sheet having no air permeability was obtained in the samemanner as in Example 1 except that a non-porous film of PET having athickness of 25 μm was used as a resin film.

Comparative Example 2

A water-proof air-permeable filter was obtained in the same manner as inExample 1 except that a PTFE porous membrane having a thickness of 15 μmand an average pore diameter of 0.8 μm was used instead of a resin film.The PTFE porous membrane was produced by firstly extruding and molding amixture of PTFE fine powder and a liquid lubricant into a sheet shape,subsequently flattening the sheet-shaped molded body by pressure anddrying the sheet-shaped molded body, and then stretching thesheet-shaped molded body by a factor of 4.5 in the longitudinaldirection and by a factor of 15 in the lateral direction at 280° C.which is lower than or equal to the melting point of PTFE.

Adhesive Force Measurement Test

Adhesive force measurement test was conducted to measure the adhesiveforce of each of the water-proof air-permeable filters and the adhesivesheet of Examples and Comparative Examples which was stuck to a housingformed of a metal plate. First, as shown in FIG. 3, the water-proofair-permeable filter or the adhesive sheet was stuck to a housing havingan opening with a diameter of 3 mm by means of the double-sided tapewhich was a component of the water-proof air-permeable filter or theadhesive sheet, such that the opening was covered from the inside of thehousing, specifically, such that the treated layer faced the outside ofthe housing via the opening of the housing. The water-proofair-permeable filter or the adhesive sheet was stuck to the housing byplacing the water-proof air-permeable filter or the adhesive sheet onthe housing, and then moving a roll having a mass of 2 kg back and forthonce over the water-proof air-permeable filter or the adhesive sheet ata speed of 5 mm/seconds.

Next, the water-proof air-permeable filter or the adhesive sheet stuckto the housing was subjected to aging for 30 minutes under a conditionof a temperature of 25° C. and a relative humidity of 65%. Subsequently,a push pull gauge (9502 manufactured by Aikoh Engineering Co., Ltd.)having a terminal with a diameter of 1.1 mm was used, and thewater-proof air-permeable filter or the adhesive sheet was pushed upfrom the outside of the housing by the terminal of the push pull gaugethrough the opening of the housing. A push-up force at the moment whenthe water-proof air-permeable filter or the adhesive sheet started tounstick was measured as an adhesive force. In addition, when thewater-proof air-permeable filter or the adhesive sheet started tounstick, the part where breakage occurred was identified.

Furthermore, in order to check the durability, the adhesive forcemeasurement was conducted also after the water-proof air-permeablefilter or the adhesive sheet stuck to the housing was immersed in a hotwater of 80° C. for 48 hours. The test results are shown in Table 1.

TABLE 1 Adhesive force (N) Part broken during Immediately after Afterhot water adhesive force sticking immersion measurement Example 1 7.916.72 Inside of double-sided tape Example 2 8.25 7.17 Inside ofdouble-sided tape Example 3 8.28 7.00 Inside of double-sided tape Com.Example 1 5.33 4.78 Inside of double-sided tape and interface betweentreated layer and double-sided tape Com. Example 2 3.32 1.81 Interfacebetween treated layer and double-sided tape

In Comparative Examples 1 and 2, breakage occurred at the interfacebetween the double-sided tape and the treated layer. This shows that thestrength of adhesion between the double-sided tape and the treated layerwas low. By contrast, in Examples 1 to 3, breakage occurred inside thedouble-sided tape. From this result, it is understood that the strengthof adhesion between the double-sided tape and the treated layer washigh. In addition, in Comparative Example 2 for which a PTFE porousmembrane was used, the adhesive force was reduced by about 45% after hotwater immersion, while in Examples 1 to 3 for which resin films wereused, the reduction of the adhesive force after hot water immersion wasless than 16%, and the adhesive force was 5 N or more even after hotwater immersion.

Water Pressure Resistance Test and Gurley Test

Next, water pressure resistance test and Gurley test were conducted foreach of the water-proof air-permeable filters of Examples andComparative Examples. First, the water-proof air-permeable filter wasjoined to the housing in the same manner as in the adhesive force test.Subsequently, water pressure is applied from the outside of the housingat 100 kPa/minute in the same manner as in JIS L1072-B (high waterpressure method), and the pressure at the moment when water started toleak from the water-proof air-permeable filter was measured as a waterpressure resistance.

In the Gurley test, the housing to which each of the water-proofair-permeable filters was joined was set to a Gurley tester specified inJIS P8117, and the time required for 100 mL of air to pass through thewater-proof air-permeable filter was measured. Subsequently, themeasurement value was converted into a value per 642 mm² to calculate aGurley number.

The results are shown in Table 2.

TABLE 2 Water pressure resistance Gurley number (kPa) (sec/100 mL)Example 1 250 22.1 Example 2 120  4.5 Example 3 100  5.8 Com. Example 2 95  6.9

It is understood from Table 2 that in Examples 1 to 3 for which resinfilms were used, both the water pressure resistance and the degree ofair permeation are high.

INDUSTRIAL APPLICABILITY

The water-proof air-permeable filter of the present invention isapplicable to, for example, outdoor lamps such as outside lights, andlamps for electric trains etc., as well as automobile electriccomponents, household electric appliances, and solar cells.

1. A water-proof air-permeable filter for ensuring ventilation and alsopreventing entry of water, the water-proof air-permeable filtercomprising: a non-porous resin film having formed therein a plurality ofthrough pores extending through the resin film in a thickness directionand each having a predetermined size larger than or equal to 0.01 μm andsmaller than or equal to 10 μm; a treated layer having hydrophobicityand oil repellency, and formed on at least one of both surfaces in thethickness direction of the resin film such that the treated layer hasopenings at positions corresponding to the plurality of through pores;and a loop-shaped double-sided tape stuck to an edge region of one ofboth surfaces in the thickness direction of the resin film, with thetreated layer interposed therebetween.
 2. The water-proof air-permeablefilter according to claim 1, wherein the plurality of through pores areuniformly distributed such that a density of the plurality of throughpores falls within specific limits included in a range from 10 to 1×10⁸pores/mm².
 3. The water-proof air-permeable filter according to claim 1,wherein a porosity of the resin film is 50% or less.
 4. The water-proofair-permeable filter according to claim 1, wherein a ratio of athickness of the resin film to the predetermined size is 1 or more and10000 or less.
 5. The water-proof air-permeable filter according toclaim 1, wherein the resin film is made of a resin that can bedecomposed by an alkali solution, an oxidant solution, or an alkalisolution containing an oxidant.
 6. The water-proof air-permeable filteraccording to claim 5, wherein the resin is at least one selected frompolyethylene terephthalate, polycarbonate, polyimide, polyethylenenaphthalate, and polyvinylidene fluoride.
 7. The water-proofair-permeable filter according to claim 1, further comprising anair-permeable supporting member laminated to a surface of the resin filmopposite to the surface to which the double-sided tape is stuck.
 8. Thewater-proof air-permeable filter according to claim 1, wherein anadhesive force when the water-proof air-permeable filter is stuck to ahousing formed of a metal plate is 5 N or more.
 9. The water-proofair-permeable filter according to claim 1, wherein when a water pressureresistance is measured in the same manner as in JIS L1072-A (low waterpressure method) or JIS L1072-B (high water pressure method) after thewater-proof air-permeable filter is stuck to a metal plate having anopening in such a manner as to cover the opening, the water pressureresistance is 1 kPa or more and 1000 kPa or less.
 10. The water-proofair-permeable filter according to claim 1, wherein when a measurementvalue is obtained by conducting measurement in the same manner as in JISP8117 after the water-proof air-permeable filter is stuck to a metalplate having an opening in such a manner as to cover the opening, and aGurley number is obtained by converting the measurement value into avalue per 642 mm², the Gurley number is 0.5 seconds/100 mL or more and500 seconds/100 mL or less.
 11. Use of a water-proof air-permeablefilter, comprising using the water-proof air-permeable filter accordingto claim 1 to cover an opening provided in a housing, and eliminate apressure difference between the inside and outside of the housing viathe water-proof air-permeable filter.